This invention relates to an internal gaseous fluid stripper for use in stripping cup-shaped articles from reciprocal rams after metal working operations have been performed on the articles. Furthermore, one of the principal uses of the internal gaseous fluid stripper concept of the present invention is in can bodymakers and the like, preferably in conjunction with a conventional external stripper. Basically, the internal stripper concept of the present invention involves the confinement within the ram prior to stripping of a predetermined volume of gas, such as air, at a predetermined pressure markedly above atmospheric pressure and freely forward flow communicating and retained in such confinement by the finished article to be stripped. Thus, as article stripping commences and ultimately proceeds to complete stripping, increasingly exposed internal volume of the article adds to the ram chamber volume permitting the confined gas to progressively expand and reduce in pressure with the initial ram chamber volume and gas pressure being predetermined relative to the finished article internal volume such that the gas mutually contained by the ram chamber and article, will continuously exert internal stripping pressure above atmospheric pressure against the article throughout the stripping operation, but will be reduced substantially to atmospheric pressure at the instant of completion of stripping. The overall result is an efficient stripping operation without collapsing or bulging the finished article during stripping even though the finished article may be quite thin-walled.
Various types of metallic cup-shaped articles have been formed by use of reciprocal rams carrying a cup-shaped article through die assemblies and ultimately requiring stripping removal from the ram after the die metal working operations are complete. A prime example thereof in modern manufacturing is the forming of thin-walled metallic can bodies as performed by modern can bodymakers. The finished can so formed is of so called two piece structure, that is, integral side and bottom walls which is ultimately closed by a top wall after filling, the one piece can side and bottom wall being termed a can body.
The first stage of forming can bodies is to blank a circular disc from sheet or strip metal followed by a first drawing operation resulting in a relatively shallow and relatively large diameter metallic cup having a wall thickness in the order of twelve to fourteen thousandths of an inch. The cup is then appropriately fed into a bodymaker wherein it is engaged by and over the end portion of a reciprocal ram just starting a forward stroke in reciprocal ram movement. During ram forward stroke, the can body is carried through a series of dies during which the can body is subjected possibly to redrawing, but at least multiple stages of wall ironing resulting in a smaller diameter, relatively deep cup-shape with a wall thickness in the order of four thousandths of an inch. At forward termination of the ram forward stroke and immediately following exiting from the wall ironing dies, the cup is forced against a bottom forming or doming die where the can bottom wall is reformed and shortly after reversal of ram movement into the ram rearward stroke, the open end of the cup side wall is engaged to essentially halt motion of the cup with continued rearward stroke motion of the ram stripping the cup therefrom.
With the increasing sophistication of can bodymakers generally operating in the manner described, production rates can be maintained in the order of one hundred-twenty to one hundred-fifty cycles per minute or combined ram forward and rearward strokes per minute. These relatively high speeds considered with the final can body extremely thin-wall thicknesses due to advances in die design have necessitated something beyond the above described mere external engagement of the can body, the use of an external stripper, in order to successfully strip the can body from the ram without undue damage to the thin side walls thereof. Thus, an internal stripping assist has been required to augment the external stripper with this internal stripping assist most usually consisting of a mechanically actuated and operable internal plunger within the confines of the ram and internally engaging the can body bottom wall. In other words, the internal stripping assist plunger is normally in a retracted position internally of the ram during the major portion of the ram reciprocal movement, but is timed for mechanical actuation to an extended position against the can body bottom wall and urging the can body from the ram at the precise moment that the external stripper is operable to engage the side walls at the can body open end.
With the extremely high ram speeds and the continuous ram movement, it can be seen that the provision of these mechanically actuated plunger-type of internal stripping assist have been necessarily quite complex and expensive to provide. Even with the ram speed at the lower production rate of one hundred-twenty can bodies per minute, this means that at least two or more can body stripping operations are performed per second and it is obvious that sophisticated timing is involved for coordinating the internal stripping assist movement of the mechanically actuated plunger with the external stripper operation, all of which is subject to wear requiring frequent maintenance refurbishment. In addition, a free internal air venting through the ram must be provided for the interior of the finished can body during stripping so as to eliminate the drawing of a vacuum within the can body during such stripping, such a vacuum not only being capable of inhibiting the relative stripping movement of the can body from the ram, but also causing a collapse of the thin-walled can body which would result in a scrap product.
Various prior attempts have been made to eliminate this relatively complex and expensive mechanically actuated plunger for internal stripping assist and replacement of the same merely with air under pressure. A stream of air under relatively high pressure in the order of ninety to one hundred pounds per square inch was provided internally of the ram and timed valve controlled to be opened against the bottom wall of the can body at the exact instant of can body engagement by the external stripper with the high pressure air stream continuing to flow throughout the stripping motion. Such attempts, however, were quickly abandoned in view of the fact that even though the force produced by the high pressure air stream at the instant of initiation of the stripping motion in order to "break" the can body from the ram to start the relative motion therebetween for such stripping, upon the almost instantaneously following completion of the stripping motion still with the internal force of the high pressure air stream caused the can body to "pop" from the ram with a considerable rearward force. The can body, therefore, exited the ram nearly as a projectal striking the doming die which it had just left so as to not only damage the can body beyond any possible use and very possibly damage the doming die requiring replacement, but otherwise presenting an extremely dangerous situation. If the air pressure was reduced sufficiently low to eliminate the "pop," then there was no force to give any stripping assist.
In an attempt to modify this air injection form of internal stripping assist to a usable form, one prior construction has made use of the same high pressure air stream internally of the ram and directed at the bottom wall of the can body, but has added thereto a valve plunger normally urged to a closed position and otherwise controlled by the bottom wall of the particular can body. It its closed position, the valve plunger extends to the bottom wall of the can body prior to bottom wall doming, thereby retaining the high pressure air stream in a closed condition throughout can body wall ironing and until the doming die is contacted at the end of the ram forward stroke. Doming of the can body by the doming die forms a recess in the can body bottom wall and during the formation of this recess, the bottom wall forces the valve plunger rearwardly within the ram to cause opening of the high pressure air stream, now directly against the can body bottom wall. As the external stripper begins the stripping action providing the opposite relative motion between the can body and the ram, as soon as the can body has been stripped from the ram the very slight doming recess distance, the valve plunger is no longer held in open position by the can body bottom wall and is permitted to move to its normally urged closed position cutting off the high pressure air stream. This means that the high pressure air is directed into the can body against the bottom wall thereof only for the very initial portion of the stripping motion and further supply thereof is then immediately sealed off.
Although this latter prior construction of internal air insertion for stripping assist does eliminate the previous problem of causing the can body to "pop" from the ram with the consequent damages and dangers involved, it still presents many prohibitive deficiencies. One very serious deficiency is that in controlling the high pressure air stream dependent on the use of the can body bottom wall, the stripping assist is only applicable to aiding in the initial breaking loose of the can body from the ram in the initiation of the relative stripping motion and for a very short period thereafter. Since the flow of pressurized air is closed off so quickly, there is not sufficient volume of air entering the can body despite the high pressure thereof to maintain a stripping assist force by the air in the latter stages of the can body stripping.
This not only eliminates completely any internal stripping assist during these latter stages of stripping, but as soon as the internal air pressure in the can body reduces to atmospheric and continues reducing therebeneath as a consequence of the steadily increasing exposed internal volume of the can body, a vacuum is drawn within the can body. As stated in the foregoing, creation of a vacuum within the can body during the stripping motion can both inhibit the stripping action and cause a collapse of the can body to damage the same beyond use. The collapse of the can body is a particularly present danger where the can body metal wall thicknesses are of the low amounts involved in order of four thousandths of an inch as produced by modern can bodymakers. Furthermore, even if the can body wall thicknesses were of sufficient strength to withstand the vacuum drawn, with the control of the volume of air being inserted being dependent on the particular doming recess in the can body bottom wall, it is apparent that this prior construction of internal stripping assist is greatly limited by the particular can body design as to the exact shape of the finished bottom wall thereof.